In general, an operable window assembly includes a sash, frame, and one or more glass panels (e.g., monolithic glass or insulating units having two or more glass panels). The one or more glass panels are mounted within the sash adjacent the inner surface of the sash, and the sash is mounted within the frame adjacent the inner surface of the frame. The frame is then mounted into an opening of a building such that the outer surface of the frame is adjacent a wall of the building that defines the opening. The sash and frame may include hollow chambers extending longitudinally within the sash and the frame, and heat differentials between the exterior surfaces of the sash and frame and the interior surfaces of the sash and frame generate convection currents of air between the sash and frame and within the hollow chambers. These convection currents transfer heat from the warmer portions of the sash and frame to the cooler portions of the sash and frame, which can reduce the thermal efficiency of the window assembly. In addition, heat is transferred between the exterior of the sash and frame and the interior of the sash and frame through conduction and radiation. For example, heat is transferred via conduction between the one or more glass panels and the sash (or frame) and between the interior and exterior portions of the sash or frame and any thermal breaks joining these portions.
Therefore, there is a need in the art to provide a more thermally efficient window assembly.